1. Field of the Invention
This invention relates to the preparation of tetrahydrofuran. More specifically this invention relates to the preparation of tetrahydrofuran from butanediol and mono- and diesters thereof.
2. Description of the Prior Art
A process for the preparation of tetrahydrofuran by the catalytic dehydration of 1,4-butanediol is described in U.S. Pat. No. 2,251,835. However this process, which is conducted so that there is always a substantial amount of the reaction mixture in the liquid phase, suffers because the acid catalyst would be washed off from a support thereby shortening catalyst life unless the catalyst is constantly replaced and because a waste acid stream results.
Tetrahydrofuran can also be prepared by the catalytic oxidation of olefins to produce carboxylic esters which are then hydroformylated, hydrogenated and hydrolyzed. For example, the oxidation of propylene to allyl acetate followed by hydroformylation to produce 4-acetoxybutan-1-ol or oxidation of butenes or butadiene to produce 1,4-diacetoxy butyl derivates which can be hydrogenated to 1,4-diacetoxybutane. However, when the carboxylic esters such as, for example, the mono- or diacetate derivatives of 1,4-butanediol are subjected to strong mineral acids such as sulfuric acid for the hydrolysis/cyclization, the tetrahydrofuran produced is accompanied by a significant amount of high boiling materials and tars. Additionally in order to obtain high yields of tetrahydrofuran, it is necessary to continuously remove the acetic acid formed or dilute the system with water. German patent application No. 2,062,950 discloses the removal of acetic acid by distillation. U.S. Pat. No. 3,586,716 discloses the removal of acetic acid by use of a low boiling ether. However, the removal of carboxylic acid by distillation or dilution with large excesses of water are required to achieve high conversions and yields to tetrahydrofuran which results in low space time throughputs. Additionally, the use of a low boiling ether to extract carboxylic acids makes the purification of the tetrahydrofuran product more difficult. The strong mineral acid catalysts used in the formation of tetrahydrofuran results in the production of a waste stream, referred to herein as waste acid streams, which must be neutralized or incinerated and thereby raises a waste disposal problem.